1. Field of Invention
This invention relates to the field of electrophotographic image forming systems and power supplies used therewith.
2. Description of Related Art
Hybrid Scavengeless Development (HSD) is a process for ionographic or electrophotographic imaging and printing apparatuses designed to prevent scavenging of toner from a photoreceptor of an imaging device by subsequent development stations.
In general, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential to sensitize the photoconductive surface. The charged photoconductive surface is exposed to a light image from either a scanning laser beam, an LED source, or an original document being reproduced. This records an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed. Two-component and single-component developer materials are commonly used for development. A typical two-component developer comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single-component developer material typically comprises toner particles. Toner particles are attracted to the latent image, forming a toner powder image on the photoconductive surface. The toner powder image is subsequently transferred to a substrate such as a copy sheet. Finally, the toner powder image is heated to permanently fuse it to the substrate in image configuration.
The electrophotographic marking process discussed above can be modified to produce color images. One color electrophotographic marking process, called image-on-image (IOI) processing, superimposes toner powder images of different color toners onto the photoreceptor prior to the transfer of the composite toner powder image onto the substrate. While the IOI process provides certain benefits, such as a compact architecture, there are several challenges to its successful implementation. For instance, the viability of printing system concepts such as IOI processing requires development systems that do not interact with a previously toned image. Since several known development systems, such as conventional magnetic brush development and jumping single-component development, interact with the image on the photoreceptor, a previously toned image will be scavenged by subsequent development if interacting development systems are used. Thus, for the IOI process, there is a need for scavengeless or non-interactive development systems. For a thorough description of scavengeless development see U.S. Pat. No. 5,031,570, hereby incorporated by reference in its entirety.
Hybrid scavengeless development technology deposits toner via a conventional magnetic brush onto the surface of a donor roll and a plurality of electrode wires are closely spaced from the toned donor roll in a development zone. An AC voltage is applied to the electrode wires to generate a toner cloud in the development zone. This donor roll generally consists of a conductive core covered with a thin (50–200 μm) partially conductive layer. The donor roll is held at an electrical potential difference relative to the conductive core to produce a field necessary for toner development. A toner layer on the donor roll is disturbed by electric fields from a wire or set of wires to produce and sustain an agitated cloud of toner particles. Typical AC voltages of the wires relative to the donor roll are 600–900 Vpp at frequencies of 5–15 kHz. These AC signals are often square waves, rather than pure sinusoidal waves. Toner from the cloud is then developed onto the nearby photoreceptor by fields created by a latent image.
A problem inherent to developer systems using wires is a vibration of the wires parallel to the donor roll and photoreceptor surfaces. This wire vibration manifests itself in a density variation, at a frequency corresponding to the wire vibration frequency, of toner on the photoreceptor. Also, higher harmonics of vibration, being an integer multiple of the wire fundamental frequency, can be excited by the applied voltage frequency. Again these vibrations can cause a density variation, at a frequency corresponding to the wire vibration frequency to produce density variations that correspond to harmonic standing wave patterns, of toner on the photoreceptor. The toner density variations and the wire vibrations that cause them are lumped together into a problem with the generic name of “strobing.” More specifically, fundamental strobing is the term used to describe the vibration and print defect associated with the fundamental mode of vibration, while harmonic strobing is used to describe the defect caused by the higher harmonics. Strobing does not occur at all hardware setpoints. For instance, it can often be reduced by decreasing the amplitude of the wire voltage, or varying the donor roll speed. Also, fundamental strobing is related to the applied wire frequency in a complex manner, and both types of strobing are sensitive to the frictional properties of the toner.